Watts on sträva? how accurate

elnino23

How accurate can the watts on Strava be? i'm guessing not very. I know I have power when i need it on the flat and tonight on hills i was putting out anything between 234 and 400 ish on different segments. Cant believe thats very accurate with no real instrument measuring it. My weight is accurate on the strava profile but still. it had me peaking at 1262 at one point i cant believe that is true!

lennymc

elnino23 wrote: »

How accurate can the watts on Strava be? i'm guessing not very. I know I have power when i need it on the flat and tonight on hills i was putting out anything between 234 and 400 ish on different segments. Cant believe thats very accurate with no real instrument measuring it. My weight is accurate on the strava profile but still. it had me peaking at 1262 at one point i cant believe that is true!

Nearly as accurate as the search function

In truth it can be up to 20% off as far as I can make out compared with a powertap

elnino23

Is that all? I thought it might be more. How are they even working it out?

lennymc

elnino23 wrote: »

Is that all? I thought it might be more. How are they even working it out?

The problem is it can be over or under and doesn't take into account wind. As a tool for measuring wattage its too inconsistent. There are other threads on ot with some more info.

Raymzor

http://www.boards.ie/vbulletin/showthread.php?p=81168217

have a look at this

uphillonly

I wouldn't pay any attention to them. As Lenny said once you the same Strava segments with/without a Powertap they are all over the place.

If it makes you feel better, Strava seems to underestimate the power.

elnino23

uphillonly wrote: »

I wouldn't pay any attention to them. As Lenny said once you the same Strava segments with/without a Powertap they are all over the place.

If it makes you feel better, Strava seems to underestimate the power.

Ha ha that does make me feel better I had thought it was over estimating. The whole thing is a bit over my head, reading that other thread i'm lost already. I think I'll go back to burying my head in the sand and just going on time ;-)

TheBlaaMan

They tell you themselves if you look in the right place (Support pages)
https://strava.zendesk.com/entries/20426531-How-does-Strava-measure-power-

uphillonly

elnino23 wrote: »

Ha ha that does make me feel better I had thought it was over estimating. The whole thing is a bit over my head, reading that other thread i'm lost already. I think I'll go back to burying my head in the sand and just going on time ;-)

Real power readings do give you more info than time and are very good training tool to see how you are progressing.

I was surprised after one ride last year when I found I'd PB'd up Featherbeds as I hadn't pushed so hard. Sure enough the power data showed I had lower average power than my previous best & was down to the tailwind.

lennymc

Power meters rock! I find it very useful when training.

Zyzz

lennymc wrote: »

Power meters rock! I find it very useful when training.

When you have the money :pac::pac:

Beasty

lennymc wrote: »

In truth it can be up to 20% off as far as I can make out compared with a powertap

That vastly underestimates the potential inaccuracy. A tailwind of 20kph would reduce by around 60-70% the power needed to maintain 30kph on the flat. Add to that the potential benefit of drafting ...

A headwind of 20kph could more than double the power required to maintain 30kph

You can see the impact of the wind by playing about with the Bike Calculator

lennymc

Zyzz wrote: »

When you have the money :pac::pac:

there were a few came up recently second hand for circa 4-500 quid. well worth the money if you can afford it. Id have one over a set of blingy wheels anyday.

Zyzz

lennymc wrote: »

there were a few came up recently second hand for circa 4-500 quid. well worth the money if you can afford it. Id have one over a set of blingy wheels anyday.

I struggle to find a really good definition as to what it does exactly Obviously it measures your power output per pedal stroke but how does one use that to train and train efficiently?

Beasty

Zyzz wrote: »

I struggle to find a really good definition as to what it does exactly Obviously it measures your power output per pedal stroke but how does one use that to train and train efficiently?

That is a whole new story ...

We have had a few threads on the subject over recent years, and it's very difficult to condense the benefits into a single post. Indeed there are quite extensive books on the subject of training with a power meter, and getting the best out of one requires a lot of research or a good coach.

I guess as a start, it measures what you have done in training, and through the use of various bits of software you can establish where your weaknesses and strengths are - what you need to focus on, and measure improvement over time

I find mine give me a shortcut in training - based on your past performance you can target improvement in a realistic way - not be over ambitious but knowing what you can do and getting to that level of fitness quicker, without "wasting" a lot of training miles/time

It also allows you to monitor your fitness levels with a view to peaking at a particular time, or identifying a time when you are in peak form and can go out and give it your best

When it comes to racing, knowing what you limits are can help pace (particularly in TTs, but it can also give you a pretty good idea of what you have left in the tank if you want to make a break for example)

lennymc

As Beasty said, there is a lot too it, and I'm relatively new to it, but from what I can see, some of the benefits are that it allows you train in particular physiological zone. I can see my power profile, and it shows me where I make good power, and where i make less good power. I can then work on improving the areas that I need to improve on. (I say "I" but I mean my coach). I also find it very good for not gauging efforts in races - on a hill for example I can glance down, see what kind of power I'm putting out, and push on, or pull back if needed. I can also see what I was doing when I get dropped, or if I attack or something like that.
There are lots of other benefits during training that I'm sure I'm missing. Its also very beneficial for analysing previous rides, identifying over training, making sure your recovery spins are easy enough to actually recover, peaking for races (with software). Getting the coach and the PM were (IMHO) the best bang for buck in my cycling career so far, and I can certainly see the benefits of it. Allen and Coggan do a great book (Training and Racing with a power meter) which, even if your not using a PM goes into a lot of the theory behind it.

seamus

I do believe Strava makes an attempt to look at perceived effort as much as anything else. So if I do a segment @ 30km/h and 60% of HrMax, and the next day do it at 27km/h but at 80% HrMax, it infers that there's some reason why the effort was greater (wind, etc) and shows higher watts for day 2 than day 1 even though it was slower.

Ultimately it'll always be guesswork and no matter how many variables you pull in and how complex your calculations are they can never rival an actual powermeter.

elnino23

I just looked up Power taps on CR! they are costing more than my bike some of them! think I will give it a few years before i go down that route! or wait till I win the lottery!

uphillonly

I can also recommend the Allen & Coggan book Lenny mentioned. I'm still relatively new to the murky world of power and learning how to use it best. Where it's immediately useful is gauging your fitness & strength progress & against previous years at the same time.

Unlike time & HR, power doesn't mislead and it doesn't matter if it's hot/cold, raining, tailwind/headwind etc. that is the power you have generated. Combined with Strava to show all the previous rides on a segment I can really gauge my progress (or lack of). Strava also shows your 1s, 5s, 1m, 10m, 20m power etc which start to mean a lot more as you come to learn about power & have a lot of relevance for racing, TTs, climbing etc. The additional data is motivating & stops me riding too many junk miles. Although power metres are sometime criticised as pros ride to them & hold back rather than go for it, I often find as Beasty said, that it pushes you more. Once you've worked out your Functional Threshold Power and zones I often push harder if I can see my wattage is lower than I know I can hold.

I justified the expense as I've had the same road bike for a few years so it was an upgrade to the bike. With Powertap it'll be easy to move it to another bike should I ever get a new bike. I bought the hub & had a wheel built on it. Garmin was already compatible so no need for a new computer.

buffalo

Was just looking at a segment I covered tonight, which I did in the exact same time in April:

Apr 19, 2013 34.6km/h 160bpm 587W 0:24
May 23, 2013 34.6km/h 161bpm 729W 0:24

Somehow I don't think the watts are in any way accurate.

Raymzor

so which power equipment do you guys recommend and what did it cost. i already have garmin 500.thanks

smacl

Slightly off topic, but a question for those using power meters on a regular basis; Should power directly relate to pedalling effort, regardless of terrain? I thought it would have, but using the power output feature on my VR trainer, it seems much easier for me to sustain higher watts on the flat than going up hills. I was wondering is this a mechanical or calibration issue (e.g. more wheel slip between the tyre and brake at lower speeds), or is this something that happens in the real world, where I could look to correcting the former. I would have thought that I could use my gears such that I could keep my power steady and cadence in a reasonable range, the the computer says no.

n.b. I realise the watts on the VR aren't that meaningful in any absolute sense, but they do seem ok from a relative point of view, comparing performance over the same course.

lennymc

afaik the power reading is a direct measurement of the power you are putting out at the power meter, so, if your cycling into a gale force wind, or up a wall, or down a hill, and it says that your putting out 200 watts, your putting out 200 watts. The perceived effort (going downhill V going up hill for example) may make it feel different, but, the actual power output is still 200 watts.

Im using a powertap sl+ built into a mavic open pro rim.

Lumen

smacl wrote: »

I would have thought that I could use my gears such that I could keep my power steady and cadence in a reasonable range, the the computer says no.

In my opinion it is easier to cycle at high power at high speed (e.g. on the flat and/or with a tailwind) than at high power at low speed (e.g. uphill and/or into a headwind) because the higher crank inertial load at higher speed provides a "better" pedalling platform.

There seems to be a lack of published research on this issue, possibly caused by the fact that laboratory-based ergometers do respond the same as a bicycle drivetrain on the road.

It's discussed a bit here (PDF).

Hmmzis

The RPE going on a flat road or dow hill might also be lower due to the added cooling from the extra airflow. Going up a steep hill the cooling effect is much less due to the lower speed and your body needs to add more effort into temp regulation.

Beasty

Lumen wrote: »

In my opinion it is easier to cycle at high power at high speed (e.g. on the flat and/or with a tailwind) than at high power at low speed (e.g. uphill and/or into a headwind) because the higher crank inertial load at higher speed provides a "better" pedalling platform.

There seems to be a lack of published research on this issue, possibly caused by the fact that laboratory-based ergometers do respond the same as a bicycle drivetrain on the road.

It's discussed a bit here (PDF).

That's interesting - I always find it "easier" to record high wattage in a group, but had thought it was more psychological than any underlying "mechanical" reason.

On the other side though my best power outputs for short distances on the road are uphill (with the benefit of a downhill beforehand to get momentum). Even up to 5-10 minute intervals my best power is on hill climbs (excluding the Wattbike which gives pretty much "perfect" conditions), but I guess that's because I'm out of the saddle for most of the time in a hill climb, which changes the dynamics

What I really need is a PM to try on the Velodrome as that involves races that are the right sort of length to both push to the limit and compare with hill climbs - I wish Garmin would get their act together;)

lennymc

i think the thing with recording best power outputs on hills is that there is a consistent opposing force - if you are climbing for 5 minutes, you have to keep applying the power, whereas if your on a flat road (are any roads really flat?) where there may be slight gradient changes, its very hard (imho) to keep a relatively steady effort due to the very slight gradient changes.

For people considering a PM, if you get one you will notice that the actual power output hops all over the place. 150 - 300 -247 -91 -120 etc for no noticable reason or increase in effort.

Guill

I don;t even find the Km cycled accurate on Strava. 8.5k to work and 9.0K home everyday, same route.

Beasty

lennymc wrote: »

For people considering a PM, if you get one you will notice that the actual power output hops all over the place. 150 - 300 -247 -91 -120 etc for no noticable reason or increase in effort.

I always have mine on 3s and/or 30s moving average to smooth this out a bit (the new Garmins have a 10s option also)

lennymc

Beasty wrote: »

I always have mine on 3s and/or 30s moving average to smooth this out a bit (the new Garmins have a 10s option also)

i've never bothered to set that up. I must have a look at it actually